Field
This technology relates generally to the tanning process and, more particularly, to recovery of chrome from a tannery process.
Background Art
Tanning, sometimes referred to as “tannery process”, is the process of treating the skin of an animal to produce leather, which is more durable and less susceptible to decomposition, and which is conducive for use in clothing or furnishing product or other covering items. Traditionally, tanning used tannin, an acidic chemical compound found in oak and fir trees from which the tanning process draws its name. A tannery is the term for a place where the skins are processed, hence, the reason for why the process is also referred to “tannery process”.
Tanning leather is a process which permanently alters the protein structure of skin. Making “rawhide” (untanned but worked hide) does not require the use of tannin. Rawhide is made by removing the flesh and fat and then the hair by use of an aqueous solution (this process is often called “liming” when using lime and water or “bucking” when using wood ash (lye) and water), then scraping over a beam with a somewhat dull blade, then drying. These methods for removing the hair can also act to clean the skin prior the tanning process to allow for penetration and action of the tanning agent, so that all the steps in preparation of rawhide except drying are often precursors to the more complex process of tanning and production of leather.
Tanning can be performed with either vegetable or mineral methods. Before tanning, the skins have the hair removed as discussed above, degreased, desalted and soaked in water over a period that can last up to about approximately 2 days. To prevent damage of the skin by bacterial growth during the soaking period, biocides can be utilized. Fungicides can also be used in the process to protect wet leathers from mold growth.
An animal can be killed and skinned before the body heat leaves the tissues and the skin can be sent on to the tannery. This can be done by the tanner, or by obtaining a skin from a rendering facility or a farm where the animals are grown. Preparing hides begins by curing them with salt. Curing is employed to prevent putrefaction of the protein substance (collagen) from bacterial growth during the time lag that might occur from procuring the hide to when it is processed. Curing removes excess water from the hides and skins using a difference in osmotic pressure. The moisture content of hides and skins gets greatly reduced. In wet-salting, the hides are heavily salted, then pressed into packs for about 30 days. In brine-curing the hides are agitated in a salt water bath for about 16 hours. The steps in the production of leather between curing and tanning are collectively referred to as beamhouse operations. They include, in order, soaking, liming, removal of extraneous tissues (unhairing scudding, and fleshing), deliming, bating, drenching, and pickling.
The Chromium(III) sulfate ([Cr(H2O)6]2(SO4)3) type tannery process is regarded as an efficient and effective tanning process. Chromium(III) compounds of the sort used in tanning are significantly less toxic than hexavalent chromium. Chromium(III) sulfate dissolves to give the hexaaquachromium(III) cation, [Cr(H2O)6]3+, which at higher pH undergoes processes called oblation to give polychromium(III) compounds that are active in tanning- being the cross-linking of the collagen subunits. The chemistry of [Cr(H2O)6]3+ is more complex in the tanning bath rather than in water due to the presence of a variety of ligands. Some ligands include the sulfate anion, the collagen's carboxyl groups, amine groups from the side chains of the amino acids, as well as “masking agents.” Masking agents are carboxylic acids, such as acetic acid, used to suppress formation of polychromium(III) chains. Masking agents allow the tanner to further increase the pH to increase collagen's reactivity without inhibiting the penetration of the chromium(III) complexes.
Collagen is characterized by a high content of glycine, proline, and hydroxprolene, usually in the repeat -gly-pro-hypro-gly. These residues give rise to collagen's helical structure. Collagen's high content of hydroxyproline allows for significant cross-linking by hydrogen bonding within the helical structure. Ionized carboxyl groups (RCO2−) are formed by hydrolysis of the collagen by the action of hydroxide. This conversion occurs during the liming process, before introduction of the tanning agent (chromium salts). The ionized carboxyl groups coordinate as ligands to the chromium(III) centers of the oxo-hydroxide clusters. Tanning increases the spacing between protein chains in collagen from 10 to 17 Å. The difference is consistent with cross-linking by polychromium species, of the sort arising from olation and oxolation.
Subsequent to application of the chromium agent, the bath is treated with sodium bicarbonate to increase the pH to 4.0-4.3. This increase induces cross-linking between the chromium and the collagen. The pH increase is normally accompanied by a gradual temperature increase up to 40° C. Chromium's ability to form such stable bridged bonds explains why it is considered one of the most efficient tanning compounds. Chromium-tanned leather can contain between 4 and 5% of chromium. This efficiency is characterized by its increased hydrothermal stability of the skin, and its resistance to shrinkage in heated water.
However, one down side to the chromium tanning process is the by products that are produced that have to be safely disposed of. Byproducts of the tanning process include some small solids, water and oil. Each of these by products can contain residual chromium that is at sufficiently high levels that it requires special hazardous waste disposal in order to safely dispose of the byproduct. The lack of the ability to safely extract and dispose of larger amounts of water and the lack of the ability to recover the chromium from larger amounts of the water in order to reuse the chromium and easily dispose of the water without hazardous waste handling measures creates a burden and cost for the tannery process. The result of the lack of ability to recover larger amounts of water from the oil byproduct is the inability to recover chromium from the oil byproduct such that the chromium lever is sufficient low to safely reuse the oil, which means that the oil has to be disposed of under certain hazardous waste standards, which can be costly.